In accordance with the principles of acoustically reflective sound attenuation, a sound reflecting baffle means is provided in the sound conducting conduit in such a way that a relatively large portion of the sound wave is cancelled out by the interference of the reflecting sound wave with the incident sound wave. It should be appreciated that such baffle means may be in the form of walls disposed in the propagating path of the sound wave, or in the form of cross-sectional restrictions provided in the sound conducting conduit. Therefore, in its simplest form, an acoustically reflective silencer is comprised of a housing having a relatively large diameter cross-section and an inlet and outlet neck of reduced diameter connected to the housing.
Furthermore, silencers for use with internal combustion engines serve to smoothen the pulsating flow of the exhaust gases produced in the exhaust system by the internal combustion engine, and to reduce the muzzle noise which is generated, primarily, by the ignition noise. The problem of sound attenuation in internal combustion engines is compounded by the circumstance that the critical sound spectrum of exhaust gases is relatively broad; i.e. in the area of 50 to 1,000 Hz, which is the reason that the ordinary engine exhaust gas muffler is a component of relatively complex structure, and that in some instances a number of silencers must be connected in series if they are to cover the entire critical sound spectrum.
The effectiveness of mufflers operating on the sound reflecting principle may be further increased by the provision of exhaust gas flow control means which will result in throttling effects.
A sound reflecting muffler of the type described in the foregoing is disclosed in U.S. Pat. No. 3,191,715. In this prior art muffler, the pipes connected, respectively, to the inlet and the outlet extend transversely through the housing axis. This is to provide for the incoming exhaust gases traveling a distance that corresponds to about three times the length of the housing before exiting through the outlet of the housing. The principal portion of the exhaust gases flow along the pipe connected with the inlet and will issue from the open end of this pipe at about midway of the housing. The gases are then reflected off the housing wall, are redirected between the pipes and the housing walls and towards the inlet opening and, after reversing their flow direction once more, are directed at a location approximately midway of the muffler, into the open end of the pipe connected with the muffler outlet. A substantially smaller portion of the exhaust gases are caused to travel directly from the housing inlet to the housing outlet by way of slots provided in the pipes so as to permit expansion of pulsating exhaust gases and to reduce the flow resistance.
This prior art sound reflection type muffler is only effective within a relatively small, medium frequency range of the sonic spectrum. Furthermore, the smoothening of the pulsating gases is incomplete. The procedure of incorporating the entire housing wall into the reflection system adversely affects the attenuating value of this muffler, because a substantial portion of the exhaust noise is transmitted by way of the housing wall into the environment.
Another engine exhaust muffler of the sound reflecting type has been disclosed in German Pat. DE-PS No. 617,831. The housing of this muffler is provided with slanted boundary surfaces and is partitioned into a number of chambers which are interconnected by pipes of varying diameters. Each individual chamber is adapted to attenuate one particular, relatively small threshold range of the sound spectrum. By virtue of the particular pipe arrangement, the individual attenuating range of each chamber is somewhat enlarged above the chamber threshold frequency. Nevertheless, even in this arrangement the stepwise attenuating procedure is not being eliminated and, like in the preceding arrangement, a considerable portion of the exhaust noise is radiated from the housing walls into the environment.
The auxiliary silencer disclosed in the German Pat. DE-PS No. 626,321, too, is characterized by the partitioning of the muffler housing. In this arrangement, the only partitioning wall has inserted therein two orthogonal pipes of different length. One of the pipes is inserted at one end into the partition wall at the level of the housing inlet. This arrangement is said to provide that with increasing engine speeds, the pipe disposed oppositely of the inlet is sufficient to a diminishing degree to provide for a smooth passage of the exhaust gases, so that the gases are rerouted and forced to travel through the pipe that is not aligned with the inlet opening, which is to provide an equalizing effect in the sound attenuation. However, one drawback of this auxiliary silencing device is that the flow resistance will increase as the engine speed increases, which will adversely affect the operation of the engine. Another shortcoming of this auxiliary device is that the sound attenuating effect is limited to a very narrow frequency range.
The muffler disclosed in U.S. Pat. No. 3,469,652 is designed primarily to smoothen the pulsating exhaust gases. The muffler housing itself is tuned to the low-frequency portion of the sound spectrum and is acoustically coupled to a pair of curvilinear tubes extending between the inlet and the outlet which, in turn, are connected at a point adjacent the inlet and outlet to a curvilinear tube connecting the inlet with the outlet. While this muffler provides satisfactory smoothening of the pulsating exhaust gases, the sound attenuation achieved with this arrangement covers only a very narrow range and, like in the prior art systems described earlier, a substantial portion of the noise spectrum is radiated through the housing wall into the ambient air.